WO2005031331A1

WO2005031331A1 - Steel material surface quality examining method

Info

Publication number: WO2005031331A1
Application number: PCT/JP2004/014568
Authority: WO
Inventors: Hiroki Fujimoto; Kiyoyuki Fukui; Mikiji Suzuki
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2003-09-29
Filing date: 2004-09-28
Publication date: 2005-04-07
Also published as: CN1723389A; CN100437099C

Abstract

A method for simply and reliable examining the weldability of a steel material, for example the oxidization scale of a hot-press steel material. (i) Two measurement terminals that can be pressed under a constant pressing pressure of 8.7 to 208 N/mm2 are pressed against a member made by a hot-pressing a hot-rolled steel sheet (cold-rolled steel sheet), a constant current of 30 A or less is made to flow between the measurement terminals to measure the voltage between the measurement terminals. (ii) The steel material is subjected to a spot-welding test, and it is judged that this welding is acceptable or not. (iii) With reference to the voltage between the measurement terminals measured about the steel material judged as acceptable, a range of the voltage or resistance between the measurement terminals leading to favorable result of the spot-welding test is determined. (iv) The voltage between the measurement terminals about a steel material is measured in spot welding of actual operation, the spot weldability of this steel material is predicted and judged according to whether the measured voltage value is within the range.

Description

Description Steel surface quality inspection method

The present invention relates to a method for detecting the surface quality of a steel material. Background art

The hot press is made from a high-strength steel material by quenching it by performing a hydraulic press using a water-cooled mold on a steel sheet charged to a heating furnace and heated to about 900 ° C. This is a technology for pressing into various parts. Particularly recently, this hot press has been applied to various steel materials including thin steel sheets. Thereby, a high-strength hot-pressed member is manufactured. The steel material to be hot pressed is heated to a high temperature of about 900 ° C prior to pressing. For this reason, oxide scale is inevitably generated on the surface of the hot press member due to this heating.

When spot welding is performed on hot-pressed members with oxide scale on the surface in this way, immediately after hot pressing, Joule heat is generated in the oxide scale generation section that comes into contact with the electrode of the spot welder. Is excessive. For this reason, the electrode of the spot welding machine is easily welded to the hot press member, and the welding causes the electrode of the spot welding machine to drop off from the spot welding machine. In particular, in the automatic mouth-port welding process in which a large number of welding robots holding the spot welding machine are arranged, the production line may be frequently stopped due to the dropout of the electrode of the spot welding machine.

In addition, hot pressed parts are usually painted after spot welding. As the oxide scale separates from the base material after this coating is performed, there is a possibility that a coating defect may occur in which the coating film formed on the oxide scale generator is separated.

Therefore, until now, before performing spot welding, an example of a hot-pressed member was used. For example, by performing a descaling treatment such as a shot-plast treatment, the oxide scale present on the surface of the hot pressed member was removed. Disclosure of the invention

Since the hot press member has a three-dimensional shape instead of a flat shape such as a steel plate, for example, the impact force of the shot in the shot blast process varies inevitably depending on the location of the hot press member. For this reason, the degree of removal of the oxide scale by the shot blasting varies depending on the location of the hot pressed member. As a result, even when the shot blasting is performed, the oxide scale may partially remain on the surface of the hot pressed member. Therefore, after the shot blast treatment, it is necessary to check whether the oxide scale has been sufficiently removed from the entire surface of the hot press member.

However, hitherto, there has been no inspection means capable of easily and quantitatively evaluating the surface quality of the hot-pressed member subjected to the shot blast treatment.

For example, firstly, there can be considered an inspection method for visually evaluating the state of formation of oxide scale on the surface of the hot press member while enlarging it with a magnifying glass or the like. However, this inspection means inevitably includes variations in the evaluation by the inspecting worker, and the evaluation is not quantitatively performed. Therefore, even with this inspection means, there is a possibility that the defective product that has excessively generated oxide scale on the surface exceeding the reference level may be overlooked. Next, it is conceivable to use a detection means for extracting and cutting a part of the hot press member after the hot press, and measuring the thickness of the oxide scale generated on the cut surface by using a microscope or the like. According to this inspection method, it is possible to make a quantitative evaluation. However, this inspection means also requires a considerable amount of man-hour (working time) to cut the hot-pressed member, and that the cut hot-pressed member cannot be flowed to the downstream process. The thickness of the oxide scale actually formed on the surface of the hot pressed member flowing in the downstream process However, since it is not directly measured, there is still a problem that defective products may be overlooked.

An object of the present invention is to provide a method for inspecting the surface quality of a steel material. An object of the present invention is to provide, for example, a state of generation of oxide scale on the surface of a hot-pressed member obtained by hot-pressing a hot-rolled steel plate or a cold-rolled steel plate. Surface quality of steel that can easily, quantitatively, efficiently and predictably determine spot weldability and paintability when painting, without breaking hot press members Is to provide an inspection method.

The electrical resistance of iron, which is the base material of the hot pressed member, is much higher than the electrical resistance of the oxide scale formed on the surface of the iron. For this reason, for example, if oxide scale is present on the surface of a steel material such as a hot-pressed member, the electric resistance value of the surface of the steel material is significantly increased.

Therefore, if the difference between the electrical resistance value of the oxide scale and the electrical resistance value of iron is used, the state of oxide scale formation on the steel surface and the spot weldability and paintability of the steel material are predicted and determined. be able to.

The present invention measures the voltage between the measurement terminals by applying a current between the two measurement terminals pressed against the steel material at a constant pressing force, and performs post-treatment such as spot welding or painting on the steel material. To determine whether the result of the post-processing is good or not, based on the voltage between the measuring terminals of the steel material determined to have a good post-processing result, between the measuring terminals capable of obtaining a good post-processing result Obtain the range of the voltage value or the resistance value between the measurement terminals, and predict the result of the post-processing performed on the steel material for which the voltage between the measurement terminals is measured based on the range obtained in this manner. In the present invention, the steel material is a hot-pressed member obtained by hot-pressing a hot-rolled steel sheet or a cold-rolled steel sheet. Welding, etc. Is desirable.

In the present invention, the load is applied to the two measurement terminals pressed against the steel material. The pressure is set to a value within the range of 8. 7~ 2 0 8 N / mm 2 to the value of the current flowing in the measurement of the measuring terminal between voltage 3 OA or less may be desirable to.

According to the present invention, when the resistance value between the measurement terminals is equal to or less than a predetermined critical value of the electric resistance, the weldability is determined to be good. On the other hand, when the critical value of the electric resistance is exceeded, the welding is performed. It is desirable to determine the sex as unpleasant. In this case, the critical value of the electric resistance, the pressure in the case of the ^{P (N / mm 2) 5} 8 P- °. 5 5 (m Ω) or more 4 8 OP-. ■ ^{6 6} (m Ω) Rukoto is desirable set the value of the range.

The present invention is implemented, for example, through the following steps (a) and (b). (a) Preferably, a current is applied to the hot press member in a state where the electrode is pressed against the descaled hot press member with a predetermined range of pressing force. Measure the electrical resistance at this time, and then perform spot welding and painting. By repeating the above operation, an appropriate range of the electrical resistance value that does not cause a problem in spot weldability or paintability is determined in advance before actual operation.

() During actual operation, measure the electrical resistance of hot-pressed members before spot welding. Then, based on whether or not the measured value of the electric resistance is within an appropriate range of the electric resistance value determined in advance in step (a), the state of formation of the oxide scale on the surface of the hot pressed member is determined. It is also determined by predicting the spot weldability and paintability of steel.

Meanwhile, Japanese Unexamined 1 1 one 4 7 9 3 5 No. 4. publications by the electrical resistance 1 5 X 1 0- ⁴ Omega below the surface of the welding member, the welding heat in the welding point An invention is disclosed in which the efficiency of welding is improved by concentrating the welding. However, this publication predicts, based on the electrical resistance of the oxide scale on the surface of the hot-pressed member, the state of formation of oxide scale on the surface of the hot-pressed member, as well as the spot weldability and paintability of steel. It is not suggested at all to make a judgment.

Further, Japanese 2 0 0 0 2 7 3 6 0 9 JP, across a pair of electrodes of aluminum plated steel sheet overlapped two 1 2. 6 kgf / mm ² pressure It is disclosed that the contact resistance value between the electrodes when pressure is applied is 0.05 to 12 mΩ, and that this contact resistance value is measured to evaluate weldability. . However, the present invention actually measures the surface roughness of the aluminum plating layer. The aluminum plating layer itself has conductivity, and various coatings provided on the aluminum plating layer are eventually uniform layers. Therefore, even if the contact resistance between the electrodes is measured according to the present invention, the weldability is not directly evaluated by this measurement.

It is well known to those skilled in the art that the contact resistance before welding hardly affects the joining strength. Brief Description of Drawings

FIG. 1 is an explanatory diagram showing a schematic configuration of a resistance measuring device for performing a method according to the present invention.

FIG. 2 is an explanatory diagram showing a situation at the time of resistance measurement by the resistance measurement device. FIG. 3 is an explanatory diagram of a resistance value measured in the method according to the present invention. FIG. 4 is an explanatory diagram showing a schematic configuration of a resistance measuring device for measuring from only one side of a steel material.

FIG. 5 is a graph showing an example of the relationship between the welding current value (kA) and the electrical resistance value Rsca1e (mΩ) of only the oxide scale.

FIG. 6 is a graph showing an example of the relationship between the electric resistance value (ηιΩ) and the applied pressure (NZ mm ² ). BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the best mode for carrying out the method for inspecting the surface quality of a steel material according to the present invention will be described in detail with reference to the accompanying drawings. In the following description, the case where the steel material is a hot pressed member is taken as an example. Also, in FIG. 2, the same members (devices) as the members shown in FIG. Number

In FIG. 1, two measuring terminals 2 and 3 are attached to a device having a pressure mechanism 1 using a hair cylinder.

The pressurizing mechanism 1 may be any mechanism that can apply a constant pressing force, and is not limited to a specific mechanism. Examples of the pressing mechanism 1 include a pressing mechanism using an air cylinder, a pressing mechanism using a servomotor, and a pressing mechanism using a spring. As the pressurizing mechanism 1, it is most convenient to use a spot welding machine insulated between a pair of electrodes arranged opposite to each other.

The measuring terminals 2 and 3 may be made of a conductive metal material. An example of such a metal material is copper alloy having high electric conductivity and high strength.

The shape of the tip of the measuring terminals 2 and 3 is not particularly limited. For example, it may be formed in a flat shape, or may be formed in a curved shape having a radius of 15 R or more, more preferably 40 R or more.

As shown in Fig. 2, if the both sides of the hot pressed member 4 are pressed by the measuring terminals 2 and 3, the electrical resistance of the front and back surfaces of the hot pressed member 4 can be measured simultaneously. , Desirable.

This pressurizing mechanism 1 is connected to an air supply source (not shown). By pressing the switch (not shown), air is supplied from the air supply source to the air cylinder, whereby the portion including the measuring terminal 2 is lowered. As a result, as shown in Fig. 2, the hot press member 4 as a test material is supported in a state where it is sandwiched between the lowered measuring terminal 2 and the fixedly arranged measuring terminal 3. Is done. When the upper and lower measurement terminals 2 and 3 are separated, the measurement terminals 2 and 3 are insulated.

The measurement terminals 2 and 3 are connected to the current generator 5. The type of the current generator 5 is not particularly limited, but is preferably a device capable of generating a constant DC current of about 3 OA or less, more preferably 0.5 A to 5 A. If the generated current value is too high, Joule heat will increase. Therefore, the temperature of the hot press member 4 may be increased, which may affect the measured value of the electric resistance, and also increases the possibility of an electric shock accident.

A constant current is supplied to the hot press member 4 via the measuring terminals 2 and 3 by the current generator 5.

The value of the current supplied to the hot press member 4 is measured by the ammeter 6. The ammeter 6 may be any device that can measure the value of the current generated by the current generator 5. When the current generator 5 is provided with a current measurement function, the ammeter 6 is unnecessary.

A small amount of current is applied between the measurement terminals 2 and 3 by the current generator 5 while the hot press member 4 is pressed and supported at a constant pressure by the two measurement terminals 2 and 3.

Pressure, although particularly not requiring limitation, 8. 7 N / mm ² (when the measurement terminal tip diameter 6 mm, 2 5 kgf) from ² 0 8 N / mm 2 (measured terminal tip diameter 6 mm In this case, it is desirable to set a constant pressure between 600 kgf).

If the applied pressure is less than 8.7 N / mm ² (25 kgf), the contact state between the hot press member 4 and the measuring terminals 2 and 3 tends to change, and the measurement error increases.

On the other hand, when the applied pressure exceeds 208 N / mm ² (600 kgf), the depth of the indentation on the surface of the hot press member 4 is increased, and the commerciality of the test material is reduced. As the number of measurements increases, the amount of deformation of the measurement terminals 2 and 3 becomes excessive, and the dispersion of the measured values increases.

Pressure is more preferably 3 4. 7 N / mm ² or more 1 7 3 N / mm ² or less (1 0 0 kgf or more 5 0 0 kgf or less).

A voltage measuring device 7 is connected to the measuring terminals 2 and 3. Generally, the electric resistance value of the hot press member 4 is in the range of 0.1 mΩ to 100 mΩ. For this reason, the voltage measuring device 7 may be any voltage measuring device or voltmeter that can measure the electric resistance value within this range. The voltage between the measuring terminals 2 and 3 is measured by the voltage measuring device 7. Then, the electric resistance value of the hot press member 4 is calculated from the voltage between the measuring terminals measured by the voltage measuring device 7 and the current value measured by the ammeter 6 described above.

The electrical resistance value of the hot pressed member 4 obtained in this way is determined in advance before operation, for example, the electrical resistance value set from the relationship between spot weldability or paintability and the electrical resistance value. By comparing with the critical value, the quality of the surface of the hot press member 4 (remaining state of the oxide scale), the spot weldability or the paintability is judged.

Specifically, when the obtained electric resistance value of the hot pressed member 4 is higher than the critical value, the amount of oxide scale remaining on the surface of the hot pressed member 4 is larger than the reference value, and And it is judged that the weldability and paintability are poor. On the other hand, when the obtained electric resistance value of the hot pressed member 4 is lower than the critical value, the oxide scale has been sufficiently removed from the surface of the hot pressed member 4, and the spot weldability and the paintability have been improved. Is judged to be good.

In this way, it is possible to quantitatively measure the residual state of the oxide scale that directly affects the spot weldability and the paintability of the hot pressed member 4.

Next, the case where the spot welding property of the hot press member 4 is predicted and evaluated by implementing the method according to the present invention using this apparatus shown in FIG. 1 will be described in further detail. Note that when performing the method according to the present invention, the operation content of the inspection is the same regardless of whether the descaling process is performed on the hot press member 4 or not. The case where the descaling process is not performed on the inter-press member 4 will be described as an example. The present invention comprises the following steps (i) to (i'v). (I) 8 ^{to. 7 N / mm 2 ~ 2} 0 8 2 two measurement terminals 2 at a constant pressure can and the pressing child of N / mm ^2, 3, with press from both sides of the hot pressing member 4, Apply a constant current of 30 A or less between measuring terminals 2 and 3. At this time, measure the voltage between the measurement terminals. (ii) Spot welding is performed on the hot pressed member 4 as post-processing, and the quality of the spot welding result is determined.

(iii) Based on the measured inter-terminal voltage of the hot-pressed member 4 determined to have a good spot welding result, the measured inter-terminal voltage value at which the spot welding result becomes good or The range of the resistance value between the measurement terminals, that is, the critical value at which the spot welding result changes from good to bad, is determined.

(i V) Measured based on the range of the voltage between the measurement terminals or the resistance value between the measurement terminals at which the spot welding result is good, that is, the critical value at which the spot welding result changes from good to bad. Judgment is made by predicting the spot welding result of hot pressed member 4 whose terminal voltage was measured.

A method for setting this critical value will be described. In the inspection according to the present invention, a preliminary test for setting the critical value and a main test for predicting and determining the quality are performed.

It is desirable that the shapes of the tips of the measuring terminals 2 and 3 and the pressing force in each of the pre-test and the main test be the same as possible.

The contact area between the measurement terminals 2 and 3 and the hot press member 4 changes due to the difference in the shape of the tips of the measurement terminals 2 and 3, and the contact area between the measurement terminals 2 and 3 differs depending on the applied pressure. This is because the microscopic contact area with the press member 4 changes, so that both the shapes of the tips of the measuring terminals 2 and 3 and the pressing force may affect the electric resistance value.

As shown in FIG. 2, first, the shapes of the tips of the measuring terminals 2 and 3 and the pressing force are made constant. Next, the hot pressing member 4 is supported by the measuring terminals 2 and 3 while being sandwiched therebetween. Next, a constant current from the current generator 5 flows between the measurement terminals 2 and 3. Then, the voltage between the measurement terminals is read after the voltage is stabilized.

Where R _t . _ta i = total electrical resistance R _t between measurement terminals 2 and 3 as voltage / current between measurement terminals. _ta i is obtained.

As shown schematically in FIG. 3, the total electric resistance R _t between the measurement terminals 2 and 3. _ta ! Can be regarded as the sum of the total electric resistance values from the measuring terminals 2 and 3 to the hot press member 4, and is expressed by the following equation (1). R _t . _{ta 1} = R 1 + R 2 + R 3 + R 4 + R 5 (1)

In the formula (1), R 1 represents the specific resistance of the measuring terminal 2, R 2 represents the contact resistance of the surface on the measuring terminal 2 side by the oxide scale, and R 3 represents the specific resistance of the base material as the test material. R 4 indicates the contact resistance due to the oxide scale on the surface on the side of the measurement terminal 3, and R 5 indicates the specific resistance of the measurement terminal 3.

Here, when the electrical resistance value of the hot pressed member 4 from which the oxide scale on the surface has been completely removed using sandpaper or the like is measured in advance, the measured value is expressed by the following equation (2). This is the sum of R 3 and the specific resistances R 1 and R 5 of measurement terminals 2 and 3.

R 1 + R 3 + R 5 (2) Then, from the total electric resistance from the measuring terminals 2 and 3 to the hot press member 4 obtained by the equation (1), the mother element obtained by the equation (2) resistivity R l with resistivity R 3 of wood measuring terminal 2, 3, subtracting the sum of the R 5, (3) shown Ri by the equation, the electrical resistance of the oxide scale only R _sea i _e GaMotomu Me Can be

The more in _{R scale = R 2 + R 4} (3) hot pressing member 4 is large the amount of oxide scale on the surface, the electric resistance value R _sea i _e the acid scale only increases. For this reason, by performing the above-described preliminary test, the relationship between the degree of spot weldability and paintability of a large number of hot pressed members 4 having different surface conditions and the electrical resistance value R scaie of only the oxide scale is determined. look, the electric resistance value R _s c _a i _e the oxide scale only as a critical value, is set in advance before operation.

Thus, the preliminary test for determining the surface quality of the hot pressed member 4 in this test is completed. .

Next, by performing this test, the hot press member 4 whose surface quality is to be inspected is subjected to the same operation as described above, so that only the oxide scale is used. Resistance value R _s c _a ! _Ask for _e .

Then, the electric resistance value R _{sc of} only the oxide scale determined by this test is _The quality of the surface quality of the hot pressed member 4 is determined by comparing aj _e with a critical value R _{sea I e} determined by a preliminary test.

That is, if the electrical resistance value R _sca J _e obtained by the present test is higher than the critical value R _sc _a , _e determined by the preliminary test, the oxide scale remaining on the surface of the hot pressed member 4 It is predicted and judged that spot weldability and paintability are unsatisfactory because the value exceeds the standard value.

On the other hand, if the electrical resistance R _sea i _e obtained in this test is lower than the critical value R _sca , _e determined in the preliminary test, on the other hand, the surface of the hot press member 4 Predict that the oxide scale has been sufficiently removed and spot weldability and paintability are good.

As described above, according to the present invention, it is possible to quantitatively measure the residual state of the oxide scale, which adversely affects spot weldability and paintability, and to determine the spot weldability and paintability of the hot press member 4. The quality of sex can be determined.

In general, the electric resistance of the measuring terminals 2 and 3, especially the electric resistance of the base material made of steel, is significantly smaller than the electric resistance of the oxide scale. In addition, both the critical value and the measured value include the electric resistances R l and R 5 of the measurement terminals 2 and 3 and the electric resistance R 3 of the hot press member 4.

For this reason, the electrical resistance value of only the oxide scale R _sea ! Although different from the embodiment described below shows a test how by value of _e, electric whole from the measurement terminal 2, 3 instead of electric resistance value R _s c _ae of oxide scale only to hot press member 4 Resistance value R _t . _The critical value may be set using _ta i.

In general, the lower the electrical resistance of the surface of the hot pressed member 4 is, the more difficult it is to weld to the electrode. However, in order to lower the electric resistance, it is necessary to cause the shotplast to strongly collide with the surface of the hot press member 4 for a long time. This causes problems such as a reduction in productivity and deformation of parts due to strong shot blast. For this reason, it is not realistic to lower the critical value of the electrical resistance on the surface of the hot press member 4 too much.

Therefore, the critical resistance of the electric resistance on the surface of the hot pressed It is desirable to set the welding current to a predetermined value in the range from 10 kA to 14 kA.

As shown by the rough in FIG. 5, the critical value of the electric resistance is a predetermined value between 5 πιΩ and 25 πιΩ at a pressure of 87 N / mm ² (250 kgf).

In addition, the electric resistance value of only the oxide scale R _sca ! _e is not an absolute value

. As shown in the graph in Fig. 6, the electrical resistance R _s c _a t _{e of} only the oxide scale

(mΩ) is a value that varies depending on the magnitude of the pressing force P. As a function of the pressure ^{P (N / mm 2) R} scale = AP- Ru can be approximated by the equation of B (provided that, A, B are constants).

Therefore, if the pressure P to be measured is different, the critical value of the electric resistance value (ιη Ω) is equivalent to between 5 Paiiotaomega in pressure ^{8 7 N / mm 2 (2} 5 0 kgf) of 2 5 m Omega Depending on the applied pressure, that is, the applied pressure P (N / mm ² ) for evaluation, 58 P—. ⁵⁵ (mΩ) ≤ R _scale ≤ 4 8 0 P ^-0-6 ⁶

What is necessary is just to set as a predetermined value within the range shown by (mΩ). As described above, R _s c _ae R _t. Because it is _ta i, R t. tai may be used instead of R _sca ".

Furthermore, when the current flowing through the measuring terminals 2 and 3 is constant, it is not always necessary to use the electric resistance value as an index, and the voltage between the measuring terminals after the voltage is stabilized is used as an index. May be. In this case, the critical value of the voltage V (V) is 5 8 1? _ '' ^{5 5} or more and 4 8 0 1?-, Where the measured current is I (A). 'It may be set as a predetermined value within the range of ⁶ or less.

FIG. 4 shows another embodiment of the method according to the invention.

That is, in the embodiments described so far, the measuring terminals 2 and 3 support the hot press member 4 as the test material in a state sandwiching from both sides. However, as shown in FIG. 4, the two measuring terminals 2 and 3 may be arranged at a predetermined distance from each other, and either the front surface or the back surface of the hot press member 4 may be configured to be pressed. . This makes it possible to measure only the electrical resistance of either the front surface or the back surface of the hot press member 4. Also in this embodiment, the specific contents of the measurement operation are substantially the same as those in the above-described embodiment, and therefore, description thereof will be omitted. In FIG. 4, the same members (devices) as those shown in FIG. 1 are denoted by the same reference numerals as those in FIG.

Next, the method according to the present invention will be described more specifically with reference to examples.

Example 1

The method according to the present invention was implemented using a stationary spot welding machine as the pressurizing mechanism 1 shown in FIG. The measurement terminals 2 and 3 were insulated by removing the copper cable connecting the measurement terminals 2 and 3 to the transformer (not shown). Then, as shown in FIG. 1, the copper wires of the current generating device 5 and the copper wires of the voltage measuring device 7 were connected to the measuring terminals 2 and 3, respectively.

Setting conditions

Measuring terminal: Cr-Cu electrode Dome radius type

Original diameter 16 mm Tip diameter 6 mm (tip 4 OR) Caro pressure: 87 N / mm ² (250 kgf)

ιΑ: [k> Current 2 A

The “critical value”, which is the evaluation criterion for the surface quality of a steel sheet, is determined by using a hot pressed member 4 whose base material is a cold-rolled steel sheet with a thickness of 1.2 mm as the test material. Short circuit without pinching anything between 2 and 3 to determine resistance of measuring terminals 2 and 3; (b) Calculate electrical resistance of base material by hot press member 4 with oxide scale completely removed; (c ) Create multiple test pieces with different surface conditions by changing shot blast conditions and measure the electrical resistance. (D) Investigate the weldability of test pieces with different surface conditions. The weldability is determined to be “OK” when welding is not performed up to 1200 A, and the weldability is determined to be “NG” when welded at less than 1200 A. The critical value at this time was set to 15 mΩ in this example by repeatedly performing the operations (a) to (d).

Next, the measurement end near the scheduled welding position of the hot pressed member 4 that needs inspection Electric current was passed between the terminals 2 and 3 to calculate the electrical resistance.

The electrical resistance value of the oxide scale was calculated by subtracting the electrical resistance values of the measurement terminals 2 and 3 and the base material from the calculated electrical resistance value.

Then, the electrical resistance value of the oxide scale is compared with the critical value. If the electrical resistance value of the oxide scale is equal to or less than the critical value (15 πιΩ), it is determined that there is no problem with the spot weldability. If the electrical resistance of the oxide scale exceeded this critical value, it was judged that there was a problem in spot weldability.

For confirmation, Table 1 shows the results of an investigation of the spot weldability of the hot pressed member 4 whose surface quality was judged. The hot pressed member 4 for which the inspection result was determined to be “NG” was susceptible to welding by spot welding.

Therefore, it was confirmed that the spot weldability can be accurately inspected by the method according to the present invention.

Table 1 Based on the R sea I e resistance of the test material Based on actual welding

(ηΩ) Inspection result Confirmation result

1 111.2 NG NG

2 47.3 NG NG

3 72.3 NG NG

4 34.5 NG NG

5 22.7 NG NG

6 14.5 0Κ 0Κ

7 4.2 0Κ 0Κ

8 0.2 0Κ 0Κ Example 2

This embodiment is different from the above-described first embodiment in that a hot-pressed member obtained by hot-pressing a hot-rolled steel sheet having a thickness of 1.4 mm was used as a test material. . The present embodiment is the same as Embodiment 1 except for this point.

The results are summarized in Table 2.

Table 2

The hot pressed member 4 whose inspection result was determined to be “NG” was likely to be welded by spot welding. Therefore, it was confirmed that the spot weldability can be accurately detected by the method according to the present invention.

Industrial potential

According to the present invention, for example, the state of scale formation on the surface of a steel material, such as a hot-pressed member obtained by hot-pressing a hot-rolled steel plate or a cold-rolled steel plate, When performing welding or painting It has become possible to easily and quantitatively and efficiently determine spot weldability and paintability, and to predict and determine steel materials without breaking them. In particular, in the situation where hot presses have become widespread recently, the present invention is an invention having a very high practicality, and has a high value that can make a great contribution to technological progress in the field. It is an invention.

Claims

The scope of the claims

1. Conducting electricity between the two measuring terminals pressed against the steel material at a constant pressing force to measure the voltage between the measuring terminals, performing post-processing on the steel material, and judging the quality of the result of the post-processing The range of the voltage between the measurement terminals or the resistance between the measurement terminals from which a good result of the post-processing can be obtained based on the voltage between the measurement terminals of the steel material judged to be good in the result of the post-processing. A method for inspecting the surface quality of a steel material, comprising: determining a result of post-processing performed on the steel material for which the voltage between the measurement terminals has been measured based on the range.

2. The surface quality of a steel material according to claim 1, wherein the steel material is a hot-pressed member obtained by hot pressing a hot-rolled steel plate or a cold-rolled steel plate, and the post-processing is welding. Inspection method.

3. The inspection of the surface quality of a steel material according to claim 2, wherein the pressure is set within a range of 8.7 to 208 N / mm ² and the value of the current is set to 30 A or less.查 method.

4. If the resistance value between the measurement terminals is equal to or less than a predetermined critical value of the electrical resistance, it is determined that the weldability is good, and if it exceeds the critical value of the electrical resistance, it is determined that the weldability is poor. The method for inspecting the surface quality of a steel material according to any one of claims 1 to 3.

5. The critical value of the electric resistance is 58 P when the pressure is P (N / mm ² ). · ^{5 5} (m Ω) or more ^{4 8 OP- ° · 6 6 (} m Ω) the range surface inspection method for steel according to claim 4, wherein the value.